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7x^2+21x-28=0
a = 7; b = 21; c = -28;
Δ = b2-4ac
Δ = 212-4·7·(-28)
Δ = 1225
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:x_{1}=\frac{-b-\sqrt{\Delta}}{2a}x_{2}=\frac{-b+\sqrt{\Delta}}{2a}\sqrt{\Delta}=\sqrt{1225}=35x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(21)-35}{2*7}=\frac{-56}{14} =-4x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(21)+35}{2*7}=\frac{14}{14} =1
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